# forbidden-state-transition

> A scalar state write may cross a transition explicitly forbidden by policy.

Canonical: https://nightshade.dicksonwu.xyz/rules/forbidden-state-transition.html
Release: Nightshade 0.1.1 (v0.1.1)

| Field | Value |
| --- | --- |
| Rule ID | `forbidden-state-transition` |
| Severity | medium |
| Confidence | medium |
| Maturity | experimental |
| Selection | Audit; needs policy |
| Policy intent | required |
| Default enabled | yes |
| Category | Policy-dependent rules |
| Engine facts used | successful-paths, state-writes, constant-constraints, transition-policy |
| Release | v0.1.1 |

## Selection and scope

Selected by the default audit profile, but it stays dormant until `policies` in nightshade.json declares the matching application intent. Selecting it alone does not supply that intent.

```sh
./.nightshade/node_modules/.bin/nightshade ./contracts --detectors forbidden-state-transition
./.nightshade/node_modules/.bin/nightshade detector show forbidden-state-transition
```

The first command runs only this rule; `--detectors` takes a comma-separated list. The second prints the rule's metadata from your installed version.

Severity is review priority. It does not express exploitability or impact. Confidence is detector metadata, not a probability. All built-in rules are experimental.

Compiler scope: the engine supports the pinned profiles 0.28.0, 0.29.0, 0.30.0, 0.31.1, 0.33.0-rc.2, 0.34.0, 0.35.0-rc.1 and 0.35.0, with ZKIR2 by default. Support for this rule can be narrower; the rule documentation below states any narrower scope.

## Corpus checks

Synthetic corpus checks for this rule in the 0.1.1 proof run. A check counts only with compiler acceptance and complete analysis.

| Positive detected | Corrected quiet | Lookalike quiet | Other negative quiet | Incomplete |
| ---: | ---: | ---: | ---: | ---: |
| 2/2 | 2/2 | 1/1 | 0/0 | 0 |

No deviations were recorded for this rule.

These are behavior checks on synthetic fixtures, not a precision or recall estimate. See [evaluation](https://nightshade.dicksonwu.xyz/evaluation.md#synthetic-corpus).

## Rule documentation

From https://github.com/DicksonWu654/nightshade/blob/v0.1.1/docs/detectors/forbidden-state-transition.md at v0.1.1.

Availability: **policy-required**; selected by default, but needs
`policies.transitions`. Severity **medium**, confidence **medium**.

Reports a scalar ledger write whose modeled prior-state constants/exclusions and
next value do not exclude a forbidden transition. Policies support enum, Boolean
and numeric state; `"to": "*"` forbids any mutation from the selected prior state.

### Example and review scenario

The compiler-qualified owned [fixture](#example-1)
sets `phase` to `OPEN` without excluding `CLOSED`. Its policy is:

```json
{ "transitions": [{ "state": "phase", "from": "CLOSED", "to": "OPEN" }] }
```

Place this object under `policies` in the [configuration](https://github.com/DicksonWu654/nightshade/blob/v0.1.1/docs/configuration.md).
The [correction](#example-2) enforces:

```compact
assert(phase != Phase.CLOSED, "terminal state");
phase = Phase.OPEN;
```

Recommendation: compare the reported write with the application's transition
table and constrain the old state before mutation. Review terminal states and
all writers, including helper-mediated writes.

### False positives and limitations

Unknown prior or next values leave transitions possible; a report does not prove
that the old state is reachable across transactions. Application invariants
outside the modeled invocation can make a match intentional or infeasible.
Aggregate/collection member transitions and general transaction sequences are
outside this scalar rule. Policies provide the intended prohibition; findings
do not infer it. Missing models and exhausted budgets remain incomplete.

### Source examples

These short regression examples are included here for installed-package readers.
Rule-specific policy requirements and limits above still apply.

#### Example 1

Fixture (source: `tests/corpus/vulnerable/forbidden-transition.compact`).

```compact
pragma language_version >= 0.26.0;
export enum Phase { OPEN, CLOSED }
export ledger phase: Phase;
export circuit reopen(): [] {
  phase = Phase.OPEN;
}
```

#### Example 2

Correction (source: `tests/corpus/safe/forbidden-transition.compact`).

```compact
pragma language_version >= 0.26.0;
export enum Phase { OPEN, CLOSED }
export ledger phase: Phase;
export circuit reopen(): [] {
  assert(phase != Phase.CLOSED, "terminal state");
  phase = Phase.OPEN;
}
```

## Suppress a match

If a match is intentional, suppress it on the line before with the exact rule ID and a reason of at least eight characters:

```compact
// nightshade-disable-next-line forbidden-state-transition -- Reason this match is intentional
```

Diagnostics cannot be suppressed, and an unmatched directive for a selected rule makes analysis incomplete. See [configuration](https://nightshade.dicksonwu.xyz/configuration.md#suppress-an-intentional-match) for fingerprint suppressions and baselines.

## References

- [Rule source document at v0.1.1](https://github.com/DicksonWu654/nightshade/blob/v0.1.1/docs/detectors/forbidden-state-transition.md)
- [Catalog entry in rules.json](https://nightshade.dicksonwu.xyz/rules.json)
- [All rules](https://nightshade.dicksonwu.xyz/rules.md#policy-dependent-rules)
- [Limitations](https://nightshade.dicksonwu.xyz/limitations.md)